Conventional optical amplifiers may be operated under variable control to achieve an amplifier response that is flat over a wide gain range. Amplifiers may also be operated in constant gain mode in which the pumps are controlled such that the amplifier gain is constant with the result being that the output power closely tracks the input power.
In optical systems, it is desirable to launch a constant power per channel into the fiber. This may be achieved by operating the amplifier in a constant gain mode. In constant gain mode, any change in the input power of a constant gain amplifier produces a proportional change in the output power of the amplifier. Thus any change in the input due to change in channel count (i.e. addition and deletion of channels), will cause a proportional change in the output power of the amplifier, keeping the per channel launch power nominally constant.
While this method is useful for tracking changes in channel count, there are drawbacks. The amplifier has no means of differentiating between changes in input power caused by channel count changes and other events such as change in fiber loss, component losses etc. While these changes may be small over a single span, these changes will accumulate along multiple sections of fibers and amplifiers to a detrimental effect.
Consider a chain 100 of optical amplifiers 102-X shown in the block diagram of FIG. 1. The input power per channel to the first amplifier 102-1 is P1. The average losses of the fiber sections 104-1 to 104-N are shown as L1, L2, . . . LN. If the launch power per channel is assumed to be same at each fiber section (and same as at the output of the first amplifier), the gains G2, . . . GN of the amplifiers 102-2 . . . 102-N track the average loss of the fiber section preceding the amplifier (L1, L2 . . . LN). The time dependent variation in fiber loss given by ΔL1, ΔL2, . . . ΔLN and the variation in input power is assumed to be ΔP1.
As noted above, conventional optical amplifiers operate in a simple constant gain mode. In this case, the control circuitry keeps the gain of the amplifier at target value GCG. A control mechanism for the conventional amplifier in constant gain mode is shown in the flow chart 200 of FIG. 2. In FIG. 2, Pin is the input power to an amplifier, and Pout is the output power from the amplifier. It is assumed that the power values are in a logarithmic scale for ease of calculation.
In step 210, the target gain GCG for the amplifier is set. This value is provisioned in the constant gain mode and typically corresponds to the gain of a particular stage or multiple stages of the chain of amplifiers. In step 220, input and output powers Pin and Pout are measured and the actual gain Gmeas is calculated from the measured power values. Step 230 determines whether or not a deviation Gerror from the target gain GCG exists. If so, the amplifier's output Pout is adjusted by Gerror in step 240 and the method returns to step 220. If the deviation is determined not to exist in step 230, the output is not adjusted and the method returns to step 220.
FIG. 3 graphically illustrates the tracking of the output power of the amplifier as a function of the input power. The amplifier is assumed to be operating at a target set gain of 25 dB. As shown, the output power of the amplifier tracks the input as closely as possible.
It can be seen that substantial output power variations occur if the constant gain control is utilized. Such power fluctuations accumulate over a chain of amplifiers to a detrimental effect as pointed out above.